1. Field of the Invention
This invention relates generally to a system for measuring a traveling direction of a vehicle using a geomagnetic direction sensor and, more specifically, to a vehicular traveling direction measuring system which is capable of providing an accurate measurement of a traveling direction of the vehicle even when a vehicle body is magnetized.
2. Description of the Background Art
One of previously proposed systems for measuring a vehicle traveling direction is exemplified by, such as, Japanese First Patent Publication No. 59-100812 published on Jun. 11, 1984.
In the system of this publication, a geomagnetic direction sensor is used for measuring the traveling direction of the vehicle. The geomagnetic direction sensor has a pair of windings which are intersected in perpendicular to each other and arranged on an annular core in a horizontal posture. The geomagnetic direction sensor outputs detected voltages (output values) corresponding to geometric directional components interlinked to the respective windings.
When the vehicle makes one turn under the uniform geomagnetism, an output circle is described on a coordinate plane using coordinate positions each defined by the voltages detected at the respective windings. Accordingly, during a normal traveling of the vehicle, a direction from a center of the output circle toward a coordinate position defined by the detected voltages is derived as a traveling direction of the vehicle.
On the other hand, when the vehicle body is magnetized or when a magnetization level on the vehicle body is changed due to the disturbance of the environmental geomagnetism, the center of the output circle is caused to vary or move, thus resulting in an error in measurement of the traveling direction of the vehicle. To correct this error, one turn of the vehicle is required to derive a new center of a corresponding output circle. Specifically, the output values of the geomagnetic direction sensor are sampled during one turn of the vehicle and then averaged to derive a new or corrected center of the output circle.
In the foregoing background art, however, because a small number of the output values are sampled to be averaged even when the magnetic environment is not preferable, it is substantially impossible to derive a corrected center coordinate position of a corresponding output circle with high accuracy. Further, one turn of the vehicle has to be performed by a driver for the correction.
Still further, although it is desirable to make one turn of the vehicle as soon as possible after the vehicle body is magnetized, it is practically difficult to seek a proper place for making the vehicle turn. Accordingly, the direction measurement error remains during the traveling of the vehicle until finding the proper place for making the vehicle turn.
Further, when the foregoing correction of the center of the output circle is performed in the condition of the large disturbance of the environmental geomagnetism, such as, on a road under a high level road or among high buildings, the correction accuracy becomes less in comparison with the correction accuracy performed in the condition of smaller disturbance of the geomagnetism. Accordingly, the corrected coordinate position of the center of the output circle tends to be unreliable so that the correction itself becomes meaningless.